[Arran]

Another thing to watch for are bad solder joints on grid caps or flaky grid cap leads, those can cause all kinds of trouble. It's no wonder that most companies routed the grid leads under the chassis as soon as the second generation octal, like 6Sk7s, came out. In terms of the radial B.E.D code resistors, it isn't so much that they change value with heat so much as the lead's contact with the carbon rod becomes flaky, it's basically a wire wrapped around a carbon rod and then painted. In fact I think this is a large part of the reason why so many increase in value, not to mention the composition in the rod drifting.
Regards
Arran